It is well known to selectively etch portions of a semiconductor wafer (e.g., silicon), by forming a patterned photoresist mask on the semiconductor wafer and placing the masked wafer in a reactive gas plasma. The plasma etches the semiconductor material exposed through the patterned mask.
A plasma etch may be carried out in an evacuated reaction chamber, for example, the reaction chamber disclosed by Reinberg in U.S. Pat. No. 3,757,733, which issued on Sept. 11, 1973, which patent is incorporated by reference herein. A typical reaction chamber has a pair of spaced apart planar electrodes. In operation, a number of masked silicon wafers are placed on the flat top surface of the bottom electrode. The top electrode is lowered to about one-half inch above the wafers, the chamber is sealed, evacuated and purged. Etching gas, such as FREON 23, is introduced into the chamber, has an electrical potential is applied across the electrodes to create a plasma therebetween. After etching is completed, the system is purged, brought back up to atmospheric pressure, opened and the etched wafers removed for further processing.
The etch rate associated with plasma etching of silicon semiconductor material is known to be temperature dependent. If the temperature of any surface falls below about 15.degree. C. a polymer from the FREON 23 gas in the system deposits out on that surface and prevents further etching. If the temperature exceeds about 40.degree. C., the photoresist mask is excessively attacked. Within the workable temperature range of about 20.degree. to 40.degree. C., the etch rates on each of the different surface materials present (Si, SiO.sub.2, photoresist) vary, so temperature variations within this 20.degree. to 40.degree. C. range affect product uniformity. Additionally, because etch rates are affected by moisture in the system, it is necessary that the temperatures of the electrodes not fall below the dew point, so it is considered desirable to have the electrodes hot when the reaction chamber is opened.
A well known technique for cooling a planar electrode is to cause a coolant (e.g., water, ethylene glycol) to simultaneously flow into a plurality of concentric channels within the planar electrode and be removed at an outlet located about 180.degree. from the inlet. However, such systems reveal temperature differences between the coolant inlet and the outlet locations of about 10.degree. C. Accordingly, wafers resting upon such an electrode would be at approximately the same temperature and would exhibit the aforementioned problems.
Accordingly, there is a need for a technique for uniformly cooling electrodes in a plasma etching facility.